Which of the following statements about differentiating pituitary versus ectopic ACTH production is true?

• A. Low-dose dexamethasone suppression test reliably differentiates pituitary from ectopic ACTH production.
• B. High-dose dexamethasone suppression test or CRH stimulation often shows suppression in pituitary sources.
• C. ACTH stimulation test is the best for this differentiation.
• D. MRI of the pituitary is definitive without any functional testing.

Answer: (B)

Distinguishing pituitary ACTH production from ectopic ACTH production relies on how the ACTH axis responds to hormonal tests. Pituitary sources (Cushing disease) often retain some feedback sensitivity, so they tend to respond to high-dose dexamethasone or to CRH stimulation, whereas ectopic ACTH sources are usually more autonomous and do not show this response.

High-dose dexamethasone suppression tends to suppress cortisol production in pituitary-origin Cushing, whereas ectopic sources typically do not suppress. Similarly, CRH stimulation often increases ACTH and cortisol when the source is pituitary, but not with ectopic tumors. This is why that statement reflects the best approach to differentiate the two.

Low-dose dexamethasone suppression is mainly a screening test and cannot reliably distinguish between pituitary and ectopic causes on its own. The ACTH stimulation test is not the primary test for source differentiation; it’s used more for assessing adrenal reserve. MRI of the pituitary alone is not definitive—pituitary imaging can miss ectopic sources and may reveal incidental pituitary lesions, so functional testing guides localization and interpretation.